1. Field of the Invention
The present invention relates to bipolar integrated circuits, and more particularly to the fabrication of avalanche diodes for use as a reference voltage in such circuits.
2. Discussion of the Related Art
Avalanche diodes have been studied for a long time, and it is known, when fabricating discrete components, how to obtain satisfactory avalanche diodes. However, in the manufacture of integrated circuits, many constraints are imposed, i.e., a large number of elemental components are simultaneously fabricated and all these components have to be made using as few technological steps as possible.
So, generally, when it is desired to manufacture a buried avalanche diode isolated from the substrate, the resulting component corresponds to the equivalent schematic diagram shown in FIG. 1. This component comprises a first avalanche diode Z1 in series with a resistor R1. A second diode Z2 is disposed in parallel with the serial connection of diode Z1 and resistor R1.
In practice, resistor R1 has often a non-negligible value, for example, approximately 400-1000 .OMEGA.. Additionally, normally, diode Z2, that has an avalanche threshold higher than the avalanche threshold of diode Z1, does not conduct. However, when the avalanche current in diode Z1 increases, the voltage drop in resistor R1 increases, and when the voltage drop exceeds the difference in the avalanche voltages between diodes Z2 and Z1, diode Z2 may also avalanche.
These various drawbacks can be avoided by providing specific doping levels for the various layers forming the avalanche diode. However, when this is done, the technological manufacturing process is complicated and it is no longer possible to use conventional technologies.
European patent applications 0,314,399 and 0,017,022 disclose avalanche diodes that have a very high avalanche voltage, 100 volts or more, while the diode Z1 has an avalanche voltage of approximately 6 volts. Thus, the parasitic diode Z2 cannot be made conductive.
Despite this advantage, the diodes according to the above-noted European patent applications still have some drawbacks. As pointed out above, the manufacturing process of such a diode uses only the technological steps that are already used for the fabrication of a conventional integrated circuit. Thus, the various doping levels of this diode are imposed by technological constraints and are not optimized for this diode. Therefore, the series resistor R1 of the diode, can be approximately 100-400 .OMEGA.. Moreover, the diode has a temperature coefficient, that is, a variation of its breakdown voltage as a function of temperature, that is approximately 2.8 mV/.degree.C. These two characteristics (internal resistance and non-negligible temperature coefficient) are major drawbacks for the fabrication of a reference diode whose voltage should be accurately determined, whatever the current flow and the temperature of the integrated circuit is.
Since these two drawbacks appear, to a variable extent, in all the conventional avalanche diodes, it has been devised in the prior art to form, using an avalanche diode, circuits for supplying a reference voltage that is compensated for current and temperature variations. An example of such a circuit is illustrated in FIG. 2. This circuit is connected to the terminals of a supply voltage source, for example, between a positive voltage supply terminal Vcc and ground G. In this circuit, a first branch is formed by the serial connection of a current source I and an NPN transistor T1. A second branch includes the serial connection of an NPN transistor T2, an avalanche diode (represented in the form of an ideal avalanche diode Z in series with an internal resistor RZ), and a resistor R. The base of transistor T2 is connected to the collector of transistor T1, and the base of transistor T1 is connected to the junction between the avalanche diode Z-RZ and resistor R. The desired reference voltage Vr is provided at the terminals of the serial connection of the avalanche diode with resistor R, that is, between the emitter of transistor T2 and ground G. If Vbe is the base-emitter voltage of transistor T1, the reference voltage is given by the following equation: EQU Vr=Vz+Vbe+RzIz,
where Iz is the current flowing through the avalanche diode.
Since the base current of transistor T7 is low with respect to current Iz, one has: EQU Iz=Vbe/R,
whereby: EQU Vr=Vz+Vbe+Vbe(Rz/R),
that is: EQU Vr=Vz+Vbe(1+Rz/R).
As can be seen, the reference voltage depends upon the ratio between the resistance of the series resistor Rz of the avalanche diode and the resistance of the resistor R that defines the current in this diode. Rz generally being non-negligible with respect to R, the absolute value of the reference voltage is subject to the variations of ratio Rz/R as well as to temperature variations, with a resulting poor reproducibility of this reference voltage.